I have always found OCR technology to be behind on open source systems. I've also watched the Ocropus project since its infancy. I've tried what I've heard is the best OCR engine available for Linux, Tesseract, and have found it woefully lacking for business documents. Are there any other more promising OCR implementations? What about the even more hopeful goal for interpreting handwriting? What is possible on *nix systems in this field?

Open-source character recognition. It converts scanned images of text
back to text files. GOCR can be used with different front-ends, which
makes it very easy to port to different OSes and architectures. It can
open many different image formats, and its quality have been improving
in a daily basis.

OCR system focusing on the use of
large scale machine learning for addressing problems in document
analysis, featuring pluggable layout analysis, pluggable character recognition, statistical natural language modeling, and multi-lingual capabilities.

The OCRopus engine is based on two research projects: a
high-performance handwriting recognizer developed in the mid-90's and
deployed by the US Census bureau, and novel high-performance layout
analysis methods.

OCRopus is development is sponsored by Google and is initially
intended for high-throughput, high-volume document conversion efforts.
We expect that it will also be an excellent OCR system for many other
applications.

Tesseract is a C++ open source OCR engine. Tessnet2 is .NET assembly
that expose very simple methods to do OCR. Tessnet2 is under Apache 2 license (like tesseract), meaning you can use it like you want, included in commercial products.

... OCR is more than "only character recognition". Image handling, preprocessing - page/layout analysis to find the texts, images, tables or barcodes. For the recognition, you have to deal with different fonts, sizes and languages. This is important because to get good results you have to use dictionaries and language definitions. Finally people expect more export options than text (e.g., XML, RTF, or searchable PDF). There are some commercial options for SDKs, but they are not cheap and for free.

If you have a budget, I strongly recommend ABBYY FineReader Engine CLI for Linux. Our company has been using it in our web-application for a year and we plan to renew the license. Very good recognition quality, command-line interface, recognition in many languages.